There's nothing routine about working in space, as astronaut Mike
Fincke found out recently when he did some soldering onboard the
International Space Station.

August 16, 2004:
Richard Grugel, a materials scientist at the Marshall Space Flight
Center, watched his video monitor in disbelief. A transmission from
the International Space Station was playing. The scene: Astronaut
Mike Fincke touches the tip of a soldering iron to a wire wrapped
with rosin-core solder.

The solder, heated, became a molten blob with a droplet of rosin
clinging tight to the outside. Solder melts: that's not too surprising.
It's the behavior of the rosin that amazed. As the temperature increased,
the droplet began to spin, round and round, faster and faster, like
a miniature carnival ride.

"What a surprise," says Grugel. "I've never seen anything
quite like it."

Grugel is the principal investigator of the In-Space Soldering Investigation,
or "ISSI" for short, which Fincke was doing at the time
of the discovery. ISSI's purpose is to find out how solder behaves
in a weightless environment. This is important information for astronauts.
If something breaks during a long trip to Mars, they'll likely reach
for a soldering iron to repair it.

The solder Fincke used for ISSI is a mixture of lead, tin and rosin. The purpose
of lead and tin is to form an electrically conducting connection.
What does the rosin do?

Grugel explains: "When metals are exposed to air, they become
coated with oxides." Iron, for example, rusts: iron oxide. "One
purpose of rosin," he says, "is to wash away any oxides
before the lead and tin solidify, clearing the way for a good strong
connection."

Rosin
has another purpose, too. On Earth and in space, surface tension tends
to hold solder in awkward blobs. Rosin breaks the tension, an action
called "wetting," allowing molten solder to flow.

But, as the video shows, weightless rosin doesn't always do what's
expected. Is this a problem?

To find out, Grugel plans to slice the solder-blobs created by Fincke
and examine what lies inside. He'll be able to see whether the solder
made a clean connection to the wire. He'll also look for tiny
bubbles of vaporized rosin inside the blobs; such bubbles, which
also appear in solder joints on Earth, lessen the electrical and thermal
conductivity of the connection.

The samples will be returned to Earth by astronauts in a Soyuz capsule
or, perhaps, after the space shuttle returns to flight. The date isn't
set.

Meanwhile, Grugel and his colleagues are brainstorming, trying to
understand what causes the rosin to twirl. "We almost have it,"
Grugel says, but he's not ready to announce a solution yet. He does,
however, have some advice for astronauts bent on soldering: wear your
goggles and watch out for flying rosin.